Porous solids have made a great impact in applications including catalysis, sorption, and separations. Advanced optoelectronics applications have been proposed that would benefit from a facile method of producing large quantities of porous materials in various compositions with high degrees of three dimensional order. For example, quantum electronics and optical communications require single-mode microcavities constructed from dielectric or walls materials with adjustable composition and multidimensional periodicity. So far, the fabrication of such structures with periodicity in three dimensions and feature sizes below 1 xcexcm has remained an experimental challenge (Joannopoulos et al., Nature, 386:143-149 (1997)). Catalysis and large molecule separation processes would also benefit from more uniform porous supports that provide optimal flow and improved efficiencies.
The use of organic templates to control the structure of inorganic solids has proven very successful for designing porous materials with pore sizes ranging from angstroms to micrometers. In the case of silicates and aluminosilicates, the organic additives are molecular and lead to microporous (at least about 0.2 nm to about 2 nm pores) zeolitic structures. Larger mesopores have been obtained by using surfactant templates or emulsion droplets as templates (Kresge et al., Nature, 3:710-712 (1992); and Monnier et al., Science, 261:1299-1303 (1993)). In mesoporous solids (greater than about 2 nm to about 50 nm pores), structural order and control of pore size has been achieved by employing micellar templates of surfactant molecules as structure-directing agents in a cooperative assembly process between the organic and inorganic species used. Although these materials are not crystalline in nature, they typically possess an ordered arrangement of pores having a narrow distribution of pore sizes. The synthesis can thus be tailored to produce pore sizes between 2 nm and 10 nm in diameter. A large assortment of mesoporous metal oxides and inorganic/organic composites has become available through modifications of the surfactant-based synthesis (Huo et al., Chem. Mater., 6:1176-1191 (1994); and Tanev et al., Nature, 368:321-323 (1994)). Micellar templates have also been used to create microporous materials with inorganic frameworks similar to their mesoporous counterparts (Sun et al., Nature, 389:704-706 (1997)).
Several techniques are currently under development to achieve even larger mesoporous (diameters up to 50 nm) and macroporous (diameters greater than about 50 nm) solids with relatively narrow pore-size distributions. For example, mesopore sizes can be increased by swelling surfactant aggregates with auxiliary organic molecules (Beck et al., J. Am. Chem. Soc., 114:10834-10843 (1992)) by adjusting surfactant and co-cation concentrations (Corma et al., Chem. Mater., 9:2123-2126 (1997)), or by postsynthesis treatment of the mesoporous sieve (Khushalani et al., Adv. Mater., 7:842-846 (1995)). The condensation of a silicate network within a triblock copolymer structure and subsequent extraction of the polymer can result in periodic mesopores with 5-30 nm diameters. Macropores with diameters of a few hundred nanometers have recently been templated in inorganic solids by latex sphere dispersions in the presence of surfactants, and by oil/formamide emulsions (Imhof et al., Nature, 389:948-951 (1997)). Although these materials can have relatively narrow pore size distributions, their structural periodicity in three dimensions has been limited. Greater order has been achieved in macroporous thin silica films which were templated by surfactant-modified latex spheres deposited on a membrane as 10 xcexcm-thick colloidal crystals (Velev et al., Nature, 389:447-448 (1997)). Others have described the synthesis of macroporous polyurethane membranes by a latex-sphere templating technique, although this involved the use of organic monomers or prepolymers to form the framework. Also, others have described the preparation of periodic macroporous carbon structures by silica sphere templating wherein the voids were filled using chemical vapor deposition or an organic resin that was calcined and converted to carbon.
Materials with bimodal pore systems are of considerable interest for applications in catalysis and separations, as they combine the benefits of each pore size regime (Davis et al., Nature, 385:420-423 (1997); and Yang et al., Adv. Mater., 9:811-814 (1997)). Micropores in zeolites provide size- or shape-selectivity for guest molecules; channels in porous solids often impart the material with very high surface areas, which can increase host-guest interactions (Zhao et al., Ind. Eng. Chem. Res., 35:2075-2090 (1996)). Bimodal pore structures involving zeolites are typically prepared by supporting zeolite crystallites on membranes (Bein, Chem. Mater., 8:1636-1653 (1996)). Attempts to crystallize the walls of mesoporous MCM-41 has resulted in the formation of a material with increased catalytic activity; FTIR spectra revealed embryonic stages of tectosilicate formation, but powder X-ray diffraction (PXRD) patterns showed no crystalline features (Kloetstra et al., Chem. Commun, 23 2281-2282 (1997)).
A need exists for general methods that readily permit the formation of macroporous structures, preferably having three-dimensional ordered structures, with many possible compositions.
The present invention provides methods of forming an inorganic macroporous materials and materials formed therefrom. In one embodiment, a method involves: providing a sample of organic polymer particles; forming a colloidal crystal template of the sample of organic polymer particles, the colloidal crystal template comprising a plurality of organic polymer particles and interstitial spaces therebetween; adding an inorganic precursor composition comprising a noncolloidal inorganic precursor to the colloidal crystal template such that the precursor composition permeates the interstitial spaces between the organic polymer particles; converting the noncolloidal inorganic precursor to a hardened inorganic framework; and removing the colloidal crystal template from the hardened inorganic framework to form a macroporous material, preferably having an ordered, three-dimensional structure. Preferably, converting the noncolloidal inorganic precursor and removing the organic polymer particles occur in one step, which preferably involves calcination.
Typically, the organic polymer particles are relatively uniform in size, preferably having a particle size distribution of no greater than about 10%, and more preferably having a particle size distribution of no greater than about 5%. Preferably, the organic polymer particles are spheres. Preferred organic polymer particles are prepared from polystyrene, polymethyl methacrylate, or a fluorinated polymer. The particles can be ordered to form a colloidal crystal template by a variety of techniques, such as centrifugation, sedimentation, spin coating, evaporation, layer-by-layer growth, crystallization, or deposition in lithographic patterns. Preferably, after the colloidal crystal template is formed it is dried. This generally opens up the interstitial spaces and typically causes the particles to hold together better. If desired, the organic polymer particles can be fused together, such as by heating the particles.
The inorganic precursor composition can include one or more inorganic precursors, which can dissolved in a solvent, preferably water, an alcohol, or a mixture thereof As used herein, xe2x80x9caxe2x80x9d or xe2x80x9canxe2x80x9d refers to one or more, thereby encompassing mixtures, blends, etc. The inorganic precursor can be a low viscosity liquid.
The inorganic precursor is preferably an alkoxide or a salt. Typically, if it is an alkoxide, converting it to a hardened inorganic framework involves hydrolysis and condensation. If it is a salt, typically converting it to a hardened inorganic framework involves adding it to the colloidal crystal template in a solution and subsequently causing it to precipitate out of solution in the interstitial spaces. Precipitation can occur simply by drying (i.e., removing the solvent), which can form a film on the template as opposed to discrete particles of a precipitate, or by using anion exchange.
After the inorganic framework is formed, the organic polymer particles that form the colloidal crystal template are removed by extracting or calcining them, for example. Prior to or subsequent to this, the chemical composition of the framework can be altered if desired (e.g., a salt can be converted to an oxide, a metal oxide can be converted to a metal, and the like).
In a preferred method of the present invention, an inorganic macroporous material is prepared by: providing a sample of organic polymer particles having a particle size distribution of no greater than about 10%; forming a colloidal crystal template of the sample of organic polymer particles, the colloidal crystal template comprising a plurality of organic polymer particles and interstitial spaces therebetween; adding an inorganic precursor composition comprising an alkoxide to the colloidal crystal template in a manner to allow the inorganic precursor composition to permeate the interstitial spaces between the organic polymer particles; condensing the alkoxide to form a hardened inorganic framework; and removing the colloidal crystal template from the hardened inorganic framework to form a macroporous material.
In another preferred method of the present invention, an inorganic macroporous material is prepared by: providing a sample of organic polymer particles; forming a colloidal crystal template of the sample of organic polymer particles, the colloidal crystal template comprising a plurality of organic polymer particles and interstitial spaces therebetween; adding a salt solution to the colloidal crystal template in a manner to allow the salt solution to permeate the interstitial spaces between the organic polymer particles; precipitating the salt out of solution within the interstitial spaces; and removing the colloidal crystal template from the hardened inorganic framework to form a macroporous material. Preferably, the method further includes converting the precipitated salt to a hardened inorganic framework prior to removing the organic polymer particles.
Significantly, the methods of the present invention can be used to prepare a wide variety of macroporous materials. For example, zeolite analogues, such as a silicalite, which has a bimodal distribution of pore sizes, can be formed, as well as hybrid inorganic/organic materials, such as organic silicate.
If desired, particularly for the formation of bimodal structures (i.e., those with generally two types of pore sizesxe2x80x94macroporous and mesoporous or acroporous and microporous), a structure-directing agent and/or a surfactant can be used, which is preferably included in the inorganic precursor composition.
The present invention also provides novel macroporous materials. Although these materials are prepared using the methods described above, other methods can be envisioned to also produce such materials. For example, other templates could be used, other inorganic precursors could be used, etc.
In one embodiment, the present invention provides a macroporous zeolite analogue having a bimodal pore structure that includes micropores and substantially uniform macropores. A preferred such zeolite analogue is a silicalite.
In another embodiment, the present invention provides a macroporous material having a bimodal pore structure that includes mesopores and macropores, wherein the macropores are ordered. A preferred such material includes silica.
In yet another embodiment, the present invention provides a macroporous material that includes a metal or metal alloy framework and macropores having an average pore size of greater than about 50 nm and less than about 10 microns. Preferably, the macropores are ordered.